Hybrid electric vehicles have a power unit, a generator, an electric motor and a battery pack. The power unit may take the form of an internal combustion engine such as a gasoline engine or a diesel engine.
Such hybrid electric vehicles may be divided into parallel hybrid electric vehicles and series hybrid electric vehicles.
In parallel hybrid electric vehicles, output of an electric motor and output of an engine are used in complementary manner or alternate manner to provide drive force to vehicle drive wheels. During regenerative braking, the motor generates output that is used to charge a battery pack.
In series hybrid electric vehicles, an electric motor provides drive force to vehicle drive wheels. An engine drives a generator whose output is used to charge a battery pack for the electric motor.
In the hybrid electric vehicles, the state of charge within the battery pack is monitored. The battery pack is charged by the output of the generator or motor until the state of charge restores to a predetermined level when the monitored state of charge drops below the predetermined level.